The different fields of the photosensitive layer are illuminated in succession, wherein in each illumination step the actual exposure dose is measured by means of the light integrator and the actual exposure time is controlled so that the actual exposure dose to which a field of the photosensitive layer is exposed corresponds to a desired exposure dose.
In a photolithography system a mask pattern is imaged on a field of a photosensitive layer which is present on a substrate, for example on a silicon wafer which is used for manufacturing integrated circuits. In order to form well-defined patterns on the substrate, it is important that the photosensitive layer is illuminated with an accurately determined quantity of energy, i.e. with an exact exposure dose. During aging of the light source its intensity is decreasing. To ensure illumination with exact exposure doses, the actual exposure doses during illumination are measured by means of a light integrator. When the actual exposure dose corresponds to a desired exposure dose, exposure of the photosensitive layer is stopped. Therefore the reliability of the light integrator is important for an accurate exposure dose. A malfunction of the light integrator will result in a deviation of the actual exposure dose from the desired exposure dose.
Known methods for testing the light integrator are performed only every two weeks since they are very time consuming. Usually, the test provides a second, calibrated light integrator which measures the light integrator to be checked. When a malfunction of the light integrator is detected, possibly a multitude of lots of wafers have already been exposed with wrong exposure doses leading to a multitude of wrongly processed lots.